Electroless copper deposition

ABSTRACT

An improved electroless metal deposition solution is provided which comprises, in combination: an ion of a metal whose electroless metal deposition is desired; a complexing agent for said ion; a reducing agent for said ion; a pH regulator; and a small effective amount of an extraneous ion which has a potential dependent capacity for inner double layer absorption at a surface in contact with the solution on which said metal is electrolessly depositing. In addition, improved methods for electrolessly depositing metal, as well as enhancing the ductility of electroless metal deposits are provided.

United States Patent Frederick W. Schneble, Jr.

Oyster Bay, L.l.;

John F. McCormack, Roslyn Heights; Rudolph J. Zeblisky, Hauppauge; John D. Williamson, Miller Place, all of N.Y.

[21] Appl. No. 847,422

[22] Filed Aug. 4, 1969 [45] Patented Oct. 26, 1971 [73] Assignee Photocircuits Corporation Glen Cove, N.Y.

by said Schneble, McCormack and Zeblisky Continuation-impart of application Ser. N 0. 768,953, Sept. 23, 1968, Continuation of application SerQNo. 523,863, Feb. 1, 1966, now abandoned. This application Aug. 4, 1969, Ser. No. 847,422

[72] Inventors s41 ELECTROLESS COPPER DEPOSITION l 1 Claims, No Drawings 561 Field ofSearchmi 106/1; l17/47R, 130, 130 E, 160

Primary Examiner Lorenzo B. Hayes Attorney-Morgan, Finnegan, Durham & Pine ABSTRACT: An improved electroless metal deposition solution is provided which comprises, in combination: an ion of a metal whose electroless metal deposition is desired; a complexing agent for said ion; a reducing agent for said ion; a pH regulator; and a small effective amount of an extraneous ion which has a potential dependent capacity for inner double layer absorption at a surface in contact with the solution on which said metal is electrolessly depositing. ln addition, improved methods for electrolessly depositing metal, as well as enhancing the ductility of electrolcss metal deposits are provided.

ELECTROLESS COPPER DEPOSITION CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of U.S. application Ser. No. 768,953, filed Sept. 23,1968, which in turn is a continuation of U.S. application Ser. No. 523,863, filed Feb. 1, 1966, now abandoned.

The present invention relates to electroless metal deposition solutions and more particularly to reducing the brittleness of metal deposits produced from such solutions.

BACKGROUND OF THE INVENTION Electroless or autocatalytic metal deposition solutions are characterized by a capacity to deposit metal on a wide variety of conducting and nonconducting or insulating surfaces without the assistance of an external supply of electrons. Typically, such solutions comprise a solvent, a supply of ions of a metal to be deposited, an agent capable of reducing the ions of the metal to be deposited, a complexing agent for the ions of the metal to be deposited and a pH regulator.

Such solutions are particularly suitable for metallizing insulating substrates on surfaces which have been suitably treated to make them sensitive to the reception of electroless metal de osition. Such sensitization techniques include the well known treatment with an acidic aqueous solution of stannous chloride (SnCl,), followed by treatment with a dilute aqueous acidic solution of palladium chloride (Pdcl Alternatively, sensitization may be achieved by treating the insulating substrates with an acidic solution containing a mixture of stannous chloride and precious metal chloride, such as palladium chloride, the stannous chloride being present in stoichiometric excess, based on the amount of precious metal chloride.

Other ways of achieving sensitization of insulating substrates to the reception of electroless copper are disclosed in copending application Ser. No. 249,063, filed Jan. 2, 1963,

, ample, in the manufacture of printed circuits and the metallization of plastics generally.

The present invention has for an object the production of electroless metal having a reduced hydrogen content.

Another object is the production of ductile electroless metal deposits which are capable of withstanding bending forces.

A further object of the present invention is the provision of novel and improved processes and compositions for providing insulating substrates, including plastics, with adherent, ductile electroless metal deposits.

Still another object'is the provision of novel and improved processes and compositions for the production of reliable, accurate and mechanically rugged printed circuits.

Other objects of the invention will in part be obvious from the following description and will in part be made clear hereinafter.

According to the present invention, it has been discovered that the ductility of electroless deposited metal may be enhanced and the other objects of this invention achieved by maintaining in the electroless metal deposition solution an extraneous ion which is adsorbed at the inner layer of the electric double layer present at the surface on which deposition is occurring. The effect of the adsorbed extraneous ion is to screen from or reduce the exposure of the deposition surface to the hydrogen which is continually generated at the deposition surface by the main electroless deposition reaction,

thereby reducing the opportunity for hydrogen to be included in the electroless deposit.

By extraneous ion is meant an ion which does not interfere directly with and is not required by the main electroless deposition reaction.

Although, for clarity of description, the invention will be particularly described with reference to electroless copper deposition, which is a preferred embodiment, it should be understood that the principles of the invention are also applicable to the electroless deposition of a variety of metals of Groups VIII and 1B of the Periodic Table of Elements, including but not limited to cobalt, nickel, palladium, platinum, silver and gold.

Electroless copper solutions are capable of depositing copper without the assistance of an extemal supply of electrons. Typically such solutions comprise water, a small amount of copper ions, e.g., derived from a water soluble copper salt, a reducing agent for cupric ions, a complexing agent for cupric ions, and a pH regulator.

The selection of the water soluble copper salts for such baths is chiefly a matter of economics. Copper sulfate is preferred for economic reasons, but the halides, nitrates, acetates and other organic and inorganic acid salts of copper may also be used.

Rochelle salts, the sodium (mono-, di-, triand tetrasodium) salts of ehtylenediaminetetraacetic acid, nitrilotriacetic acid and its alkali salts, gluconic acid, gluconates, and triethanolamine are preferred as copper ion complexing agents, but commercially available glucono-rS-lactone and modified ethylenediamineacetates are also useful, and in certain instances give even better results than the pure sodium ethylenediaminetetraacetates. One such material is N-hydroxyethylethylenediaminetriacetate. Other materials suitable for use as cupric complexing agents are disclosed in U.S. Pat. Nos. 2,996,408; 3,075,856; 3,075,855 and 2,938,805.

Preferred reducing agents for cupric ion which have been used in alkaline electroless metal baths include formaldehyde, and formaldehyde precursors or derivatives, such as paraformaldehyde, trioxane, dimethyl hydantoin, glyoxal, and the like. Also suitable as reducing agents in alkaline baths are borohydrides, such as alkali metal borohydrides, e.g., sodium and potassium borohydride, as well as substituted borohydrides, e.g., sodium trimethoxy borohydride. As reducing agents in such baths may also be used boranes, such as amine borane, e.g., isopropylamine borane, morpholine borane, and the like.

Typical of the reducing agents for cupric ion in acid electroless copper solutions are hypophosphites, such as sodium and potassium hypophosphite, and the like.

Reducing agents of the type described will release hydrogen.

The pH adjuster or regulator may consist of any acid or base, and here again the selection will depend primarily on economics. For this reason, the pH adjuster on the alkaline side will ordinarily be sodium hydroxide. On the acid, side, pH will usually be adjusted with an acid having a common anion -with the copper salt. Since the preferred copper salt is the sulfate, the preferred pH adjuster on the acid side is sulfuric acid. 1

In normal operation of the bath, the copper salt serves as a source of cupric ions,- and the reducing agent reduces the cupric ions to metallic form. When reducing agents of the type described above are oxidized to provide electrons for the reduction of the cupric ions, hydrogen is released at the site of deposition. The complexing. agent serves to complex the cupric ion, and at the same time makes the cupric ion available as needed to the reducing action of the reducing agent. The pH adjuster serves chiefly to regulate the internal plating potential of the bath.

It should be understood, however, that every constituent in the electroless copper bath has an effect on plating potential, and therefore must be regulated in concentration to maintain the most desirable plating potential for the particular ingredients and conditions of operation. Other factors which affect internal plating voltage, deposition quality and rate include temperature and degree of agitation, in addition to type and concentration of the basic ingredients mentioned.

ln electroless plating baths, the bath constituents are continually being consumed, so that the bath is in a constant state of change.

Electroless copper solutions having the basic constituents described produce copper deposits which contain a substantial amount of included hydrogen. Such deposits are brittle, break under vibration and bending, and otherwise exhibit poor ductility. Such deposits also exhibit a dull surface, of poor color, which may best be described as smutty."

The electroless copper of this invention, as distinguished from that of the prior art, has a greatly reduced hydrogen content, is bright, and possesses enhanced ductility, as compared to the electroless copper of the prior art.

The precise mechanism by which electroless metal, e.g., copper, deposition occurs is complex and difi'icult of precise definition. Without wishing to be limited thereto, the ensuing theory affords a rational explanation for this phenomenon.

In electroless copper deposition from a solution containing a solvent, a source of cupric ions, a reducing agent for cupric ions, a complexing agent for cupric ions, and a pH regulator, the reducing agent is oxidized at deposition surface areas to release electrons to such areas. Cupric ions in contact with such areas pick up the free electrons and are reduced to copper atoms, which deposit on the adjacent surface areas and aggregate to form a copper metal deposit.

The potential at which the process takes place is a compromise, or mixed potential, between the two potentials which characterize the oxidation of the reducing agent and the reduction of the cupric ions. The potential of operation of the bath, the mixed potential, is between the potential which characterizes the reduction of the cupric ions in the absence of the reducing agent and the potential which characterizes the oxidation of the reducing agent in the absence of the cupric ions. The former is between 0.2 and -0.5 and the latter between 0.9 and -l.3 volts versus the saturated calomel electrode depending on temperature and composition of the bath. Thus, the adsorbed species must have the property to be adsorbed at the potentials between 0.2 and l'.3 volts versus the saturated calomel electrode.

To illustrate the mechanism, in one embodiment of a particularly useable electroless copper deposition solution, the reducing agent is formaldehyde, and the source of the cupric ions is a copper salt, such as cupric sulfate. Using the mixed potential theory heretofore described, the formaldehyde is oxidized to hydrogen and formate ions, or to hydrogen and car bon dioxide, at the sensitized surface at which deposition is occurring, Piersma and Gileadi, Modern Aspects of Electrochemistry," Vol. 4, p. 13 l, and one electron is given up to the surface for each molecule of formaldehyde oxidized. The released electrons are in turn taken up by cupric ions at or in contact with the surface, to cause local deposition of copper.

The surface on which electroless metal deposition occurs may be considered a charged interface having the structure of an electric double layer. For a description of the electric double layer structure of such a charged interface, see: Proc. Roy. Soc., Series A, I963, pp. 55-79; and Bockris, Modern Aspects of Electrochemistry No. l, 1954, pp. l03-l73, which are herein incorporated by reference as part of the instant specification.

According to the present invention, the inner layer of the charged interface or surface at which deposition occurs, has adsorbed thereon extraneous ions, as described above, which screen the surface at least in part from the adsorption of hydrogen released by the reducing agent. To accomplish this result, there are maintained in the electroless copper solution, particularly one having a high plating potential, extraneous ions which are capable of being preferentially adsorbed at the inner layer of the deposition surface. The adsorbed, extraneous ions must be potentially dependent, i.e., they must have the capacity to be adsorbed at the potential of operation of the bath, whatever that potential may be. They must also have the capacity to be preferentially adsorbed, compared to the ions normally present and required for successful operation of the bath.

The extraneous ions suitable for use must also, as stated previously, have the capacity of being specifically adsorbed at the inner layer of the electric double layer present at the surface on which deposition is occurring. The inner layer of the electric double layer present at the charged interface has a thickness approximately equal to one to two molecular diameters, or about 3 Angstroms. The capacity of the extraneous ions suitable for use herein to be adsorbed at the inner layer of the surface on which deposition is occurring distinguishes such ions from those ions which may be electrostatically or coulombically adsorbed at the outer layer of the electric double layer. The outer layer of the electric double layer may be considered as being separated from the inner layer by a distance of about one to two molecular diameters, or about 3 Angstroms.

The difference between inner layer and outer layer adsorption is significant. ln inner layer adsorption of the extraneous ion according to this invention, there is a specific bonding of the extraneous ions to the surface, i.e., the extraneous ions are chemisorbed on the surface. The specific bonding of the extraneous ions at the inner layer of the surface on which deposition is occurring reduces the repulsion force between such ions, thereby permitting better coverage of the surface, and, consequently, better screening of the deposition surface from hydrogen adsorption.

In other words, the specific adsorption of the extraneous ions to the deposition surface reduces the net repulsive charge on the ions, thereby permitting more extraneous ions to be absorbed on the surface, which, in turn, leads directly to elimination or reduction in hydrogen adsorption at the electric inner layer.

The adsorbed extraneous ions are highly mobile and move freely but randomly within the plane of the inner layer, which is parallel to the plane of the charged interface. Because of the electrical forces bonding them to the surface, however, such ions move considerably less freely in directions which are vertical to the surface.

The adsorbed extraneous ions of this invention compete for space in the inner layer with hydrogen which also has a capacity for inner layer adsorption. Accordingly, the greater the number of extraneous ions adsorbed at the inner layer, the less space available for inner layer hydrogen adsorption. A decrease in the number of hydrogen ions adsorbed at the inner layer leads directly to a decrease in hydrogen inclusion in the electroless metal and a consequent increase in the ductility of the deposited metal.

Among the extraneous ions having a potential dependent, preferential capacity for inner layer adsorption at the deposition interface, and therefore suitable for use herein, are ions which contain one or more elements selected from the group consisting of vanadium, arsenic, antimony, bismuth and mixtures of the foregoing.

Such extraneous ions may take any number of specific forms.

Suitable sources of vanadium and arsenic containing ions are the oxides of such elements, as well as organic and inorganic acid water soluble salts of such elements, e.g., the vanadates and arsenates of the metals of Groups 1A and "A of the Periodic Table of Elements, and ammonium. Preferred for use are the sodium, potassium and ammonium salts. Sources of antimony and bismuth containing ions are the oxides of such elements and water soluble organic and inorganic acid salts of such elements, including the sulfates, nitrates, halides, tartrates, and the like.

The foregoing sources are merely typical of those which are capable of providing extraneous ions which have a potential dependent, preferential capacity for being adsorbed at the inner layer of the electric double layer which is present at the deposition interface.

hydroxide. to give The extraneous ions described herein should be maintained in the electroless metal solutions in small effective amounts. Ordinarily, their concentration will average between about 0.1 and 10,000 microgram mole per liter of solution, preferably between about i and 800 microgram mole per liter. As used herein, a microgram mole is l l0 gram mole.

It should be emphasized, however, that the small effective amount of extraneous ion will vary with the nature and activity of the ion used, and with makeup of the solution and the conditions, e.g., the temperature, concentration and pH, under which it is used. The upper limit of extraneous ion is an amount which will prevent deposition of electroless metal under conditions of use. The lower limit is the least amount of ion which will be effective in manifesting the results described herein, again under the particular conditions of use.

Here it should be noted that excess amounts of extraneous ions of the type described may stop the bath completely under certain conditions of use. So sensitive is the concentration on some of the ions that amounts measured in parts per million may stop the bath completely and practically instantaneously at a given activity level, as controlled by a given temperature and given reactant concentrations and types.

A typical electroless metal deposition bath made according to the present invention will comprise: 2 5

Electroless metal salt 0.002 to 1.0 mole Reducing agent 0.03 to 4 moles Electroless metal complexing agent 07 to 40 times the moles of metal salt Extraneous ion 0.] to 10,000 microgram mole pH adjuster sufficient to give desired pH Water suflicient to make 1 liter,

Embodiments of a high plating potential electroless copper solution comprise:

Reducing agent. preferably formaldehyde Extraneous ion 0.03 to 13 moles l to 800 microgram moles Cupric ion complexing forth herein, it should be understood that as the baths are used up in plating, the ingredients will be replenished from time to time. Also it is advisable to monitor the pH, and the concentration of the extraneous ions, and to adjust them to their optimum value as the bath is used.

For best results, surfactants in an amount of less than about 5 grams per liter may be added to the baths. Typical of suitable surfactants are organic phosphate esters and oxyethylated sodium salts.

Particularly, results are achieved when the extraneous ions of this invention are added to or maintained in solutions which contain small effective amounts, e.g., between 5 micrograms and 500 milligrams per liter, of water soluble cyanide compounds. Typical of such cyanide compounds may be mentioned alkali cyanides, such as sodium and potassium cyanide; nitriles, such as chloroacetonitrile; alpha-hydroxy nitriles, e.g., glyconitrile and lactonitrile; and dinitriles, e.g., succinonitrile, iminodiacetonitrile, and 3,3 iminodipropionitrile.

The baths may be used at widely varying temperatures, e.g., between 15 and 100 C., although they will usually be used between about 20 and 80 C. As the temperature is increased, it is usual to find that the rate of plating is increased, but the temperature is not highly critical, and, within the usual operating range, excellent bright, ductile deposits of electroless copper of reduced hydrogen content are obtained.

Performance data for baths made in accordance with the teachings contained herein are given in Table I.

TABLE I.EXTRANEOUS ION Tetrasodium ethylene- Potassium diamine HCHO antimony Thickness of CuSOr 5H2O ten-ascetic 37% V205 NtJASO2 tartratc deposit gm. acid (gun/1.) (mL/l.) (gm./l.) (gn1./l.) (gm./l.) Stability (inch) 40 6 Unstable... 00538 40 6 Stable .00047 Copper salt 0.002 to L0 mole Reducing agent 0.03 to 4 moles Cupric ion complexing agent 0.7 to 40 times the moles of copper Extrancous ion 0.! to l0,000

microgram mole pH adjuster sufficient to give desired pH Water sufficient to make I liter.

Preferred embodiments of highly active electroless copper solutions comprise:

A soluble cupric salt. preferably cupric sulfate 0.002 to 0.6 mole Alkali metal hydroxide. preferably sodium pH of l0-l4 In Table I, the solutions were maintained at a pH of about 12 and at elevated temperature throughout use. In all instances about 1 ml./l. of an organic phosphate ester was used as a surfactant.

In Table I, ductility is measured by bending the copper deposit through in one direction, creasing, then returning it to its original position, with pressing along the crease to flatten it, this cycle constituting one bend.

As shown in Table l, the presence of an extraneous ion enhances the ductility of the copper deposits to a truly remarkable degree.

Use of the extraneous ions described also improves stability of the electroless copper solution to a marked degree, as is also brought out in Table I.

The ductile electroless metal of this invention, in addition to having a reduced hydrogen content, is characterized by the presence of one or more of the elements making up the extranepus ions, i.e., vanadium, arsenic, antimony, bismuth, and mixtures of the foregoing.

In using the electroless copper solutions to plate metal, the surface to be plated must be free of grease and other contaminating material.

Where a nonmetallic surface is to be plated, the surface area to receive the deposit must first be sensitized to render it catalytic to the reception of electroless copper, as brought out hereinabove.

Where metal surface is to be plated, it should be degreased, and then treated with an acid, such as hydrochloric or phosphoric acid, to free the surface of oxides.

Following pretreatment and/or sensitization, the surface to be plated is immersed in the autocatalytic metal baths, and permitted to remain in the bath until a metal deposit of the desired thickness has built up.

As has been brought out heretofore, the solutions described herein are advantageous for use in the production of printed circuits. For example, portions of the surface of an insulating substrate in the form of a desired circuit pattern may be sensitized for the reception of electroless metal. Following sensitization, the substrate is immersed in the electroless metal solution of the type described and permitted to remain therein until a metal deposit of the desired thickness has been built up. The circuit may be formed on one or more surfaces of the substrate. If desired, interconnections between the surfaces may be provided by drilling or punching holes and sensitizing the lateral walls thereof prior to exposure of the substratum to the electroless metal solution. In this embodiment, electroless .metal builds up on the circuit pattern, as well as on the walls surrounding the holes to form interconnections.

To enhance the stability of the electroless metal solutions described herein, certain sulfur containing compounds may be maintained therein during operation.

'Aifibhgih organic 551E compounds may be mentioned the following: aliphatic sulfur-nitrogen compounds, such as thiocarbamates, e.g., thiourea; S-membered heterocyclics containing S-N in the -membered ring, such as thiazoles and isothiazoles, e.g., Z-mercapto benzol thiazole and the like; dithiols, e.g., 1,2-ethanedithiol and the like; 6-membered heterocyclics containing S-N in the ring, such as thiazines, e.g., 1,2-benzisothiazine, benzothiazine, and the like; thioamino acids, such as methinonine, cystine, cysteine, and the like; thio derivatives of alkyl glycols, such as 2,2 thiodiethanol, dithiodiglycol, and thioglycollic acid; and the like. Among the inorganic sulfur compounds may be mentioned: alkali metal sulfides, e.g., sodium sulfide, potassium sulfide, sodium polysulfide, potassium polysulfide; alkali metal thiocyanates, such as sodium and potassium thiocyanates; and alkali metal dithionates, such as sodium and potassium dithionate.

The foregoing sulfur compounds are merely typical of sulfur compounds which are capable of stabilizing autocatalytic copper baths as taught herein.

The amount of sulfur compound required is a small effective amount and will vary, depending upon the particular compound used, from a trace to about 300 parts per million (p.p.m.) or more. For most sulfur compounds, 1 p.p.m. will be the upper limit and about 0.001 p.p.m. the lower limit. A good working limit for most sulfur compounds is between about 0.01 and 0.2 p.p.m.

Solutions containing a small effective amount of sulfur component are especially useful in the preparation of printed circuits. In the absence of sulfur, there is a tendency with electroless solutions of the type described for nonsensitized areas of the insulating substrates, following prolonged immersion in the solution, to become sensitized and to receive scattered spot deposits of metal. As will be readily appreciated, deposition of electroless metal in areas of the substrate where metal is not desired would raise havoc with control techniques in the preparation of printed circuits.

Additionally, when autocatalytic metal baths are used commercially, there is a tendency for extraneous metal to deposit on insulating walls of containers housing the electroless metal solution for prolonged periods of time.

The sulfur containing baths of the present invention are re markable in the sense that they will deposit electroless metal on nonmetallic surfaces only in those areas which have been sensitized to provide catalytically active sites, as described for example hereinabove. These baths have a remarkable capacity for distinguishing nonmetallic areas which have not been so sensitized from those which have, and for depositing metal only on the latter areas. Even though a nonpretreated, nonmetallic surface, including the housing walls of the baths, may be exposed to the baths of this invention for prolonged periods of time, the tendency for scattered spot deposits of metal to form is substantially reduced, if not completely eliminated.

It should also be pointed out that the extraneous ions described herein are especially suitable for use in electroless copper solutions which are run on a substantially iron-free ba- SIS.

According to copending application Ser. No. 523,902, of Frederick W. Schneble, .lr., John F. McConnack and Rudolph J. Zeblisky, filed Feb. I, 1966, the reduction or substantial elimination from electroless metal deposition solutions of ions which have an oxidation potential greater than the oxidation potential of the ion of the metal sought to be electrolessly deposited also alleviates the problem of spontaneous decomposition ordinarily associated with such solutions. More specifically, the copending application suggests maintaining the concentration of such ions below about 25 parts per million.

The expression oxidation potential used herein should be understood as having the definition set forth in Latimer, Oxidation Potentials, 2nd Ed. Prentice Hall, 1952.

In electroless copper solutions, ions of iron are particularly troublesome in this respect, and the concentration should b? maintained below about 25 parts per million.

The extraneous ions of this invention do not react with iron, and therefore do not interfere with removal of iron from electroless copper solutions. Rather, with the extraneous ions of this invention, the electroless copper solution may be continuously or periodically treated to remove deleterious ions of the type described in the preceding paragraph, as by raising the pH of the solution to thereby precipitate iron as the hydroxide.

The invention in its broader aspects is not limited to the specific steps, processes and compositions shown and described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed:

I. An electroless copper deposition solution which comprises a water soluble copper salt, a reducing agent for cupric ion, a complexing agent for cupric ion, and a pH adjuster, the improvement which comprises having present in such solution an amount of an extraneous ion which comprises an element selected from the group consisting of vanadium, arsenic, antimony, bismuth and mixtures of the foregoing.

2. In a solution of claim I, in which said extraneous ion is adsorbable at the inner layer of an electric double layer present at a surface in contact with the solution of said copper is electrolessly depositing.

3. In the solution of claim 1, in which said extraneous ion is chemisorbable at the inner layer of an electric double layer present at a surface in contact with the solution on which said copper is electrolessly depositing.

4. An electroless copper deposition solution of claim 1 which includes a water soluble cyanide compound.

5. An electroless copper deposition of claim 1 which includes a sulfur containing compound.

6. An electroless copper deposition solution of claim 1 which includes a cyanide compound and a sulfur compound.

7. The solution of claim 1 which includes less than 25 parts per million of an ion having an oxidation potential greater than the oxidation potential of the ion whose electroless deposition is desired.

8. The solution of claim 7 wherein said less than 25 parts per million of an ion is iron ions. V

9. The solution of claim 1 wherein the amount of said extraneous ion is between about 0.1 and 10,000 microgram mole per liter.

10. In a method for electrolessly depositing metal which comprises contacting a surface sensitive to the reception of electroless copper with an electroless metal deposition solution containing an ion of copper, a complexing agent for said ion, a reducing agent for said ion and a pH regulator, the improvement which comprises maintaining in the solution an effective amount of an extraneous ion of an element selected from the group consisting of vanadium, arsenic, antimony, bismuth and mixtures thereof which extraneous ion has a potential dependent capacity for being preferentially adsorbed at the inner layer of the electric double layer present at the electroless metal deposition surface for screening said surface from the adsorption of hydrogen released by said reducing agent.

11. In a method for depositing copper on designated areas of an insulating member to form printed circuits, the surface of said designated areas being sensitive to the reception of electroless copper by contacting the insulating member with an electroless copper deposition solution; the improvement for enhancing the ductility of the electroless copper deposited on said designated, sensitized areas which comprises maintaining in the solution an extraneous ion having a potential dependent capacity for being adsorbed on said designated areas in preference to hydrogen, said extraneous ion being of an element selected from the group consisting of vanadium, arsenic, antimony, bismuth and mixtures thereof. 

2. In a solution of claim 1, in which said extraneous ion is adsorbable at the inner layer of an electric double layer present at a surface in contact with the solution of said copper is electrolessly depositing.
 3. In the solution of claim 1, in which said extraneous ion is chemisorbable at the inner layer of an electric double layer present at a surface in contact with the solution on which said copper is electrolessly depositing.
 4. An electroless copper deposition solution of claim 1 which includes a water soluble cyanide compound.
 5. An electroless copper deposition of claim 1 which includes a sulfur containing compound.
 6. An electroless copper deposition solution of claim 1 which includes a cyanide compound and a sulfur compound.
 7. The solution of claim 1 which includes less than 25 parts per miLlion of an ion having an oxidation potential greater than the oxidation potential of the ion whose electroless deposition is desired.
 8. The solution of claim 7 wherein said less than 25 parts per million of an ion is iron ions.
 9. The solution of claim 1 wherein the amount of said extraneous ion is between about 0.1 and 10,000 microgram mole per liter.
 10. In a method for electrolessly depositing metal which comprises contacting a surface sensitive to the reception of electroless copper with an electroless metal deposition solution containing an ion of copper, a complexing agent for said ion, a reducing agent for said ion and a pH regulator, the improvement which comprises maintaining in the solution an effective amount of an extraneous ion of an element selected from the group consisting of vanadium, arsenic, antimony, bismuth and mixtures thereof which extraneous ion has a potential dependent capacity for being preferentially adsorbed at the inner layer of the electric double layer present at the electroless metal deposition surface for screening said surface from the absorption of hydrogen released by said reducing agent.
 11. In a method for depositing copper on designated areas of an insulating member to form printed circuits, the surface of said designated areas being sensitive to the reception of electroless copper by contacting the insulating member with an electroless copper deposition solution; the improvement for enhancing the ductility of the electroless copper deposited on said designated, sensitized areas which comprises maintaining in the solution an extraneous ion having a potential dependent capacity for being adsorbed on said designated areas in preference to hydrogen, said extraneous ion being of an element selected from the group consisting of vanadium, arsenic, antimony, bismuth and mixtures thereof. 